Phytoplankton of the York River
Harold G. Marshall
Old Dominion University
Norfolk, VA 23529 U.S.A.
The York River possesses a diverse phytoplankton community represented by a variety of algal species that includes both freshwa-
ter and estuarine flora. The mean annual monthly range of abundance is ca. 5-20 X 106 cells L-1 with an extended bi-modal pat-
tern that begins with an early spring diatom peak (March) that declines into early summer. The development of a more diverse
representation of taxa in the summer results in a secondary late summer-early fall peak. Diatoms are the dominant phytoplank-
ton component throughout the entire estuary including a variety of pennate and centric species such as Asterionella formosa and
Aulacoseira granulata. Dinoflagellates are more common and abundant in the lower segments of the York River where they have
been associated with re-occurring and extensive “red tide” blooms. These include Cochlodinium polykrikoides, Heterocapsa triquetra,
Heterocapsa rotundata, Scrippsiella trochoidea, and Prorocentrum minimum. Cynobacteria, commonly referred to as blue-green algae,
include unicellular, colonial, and filamentous taxa that are predominantly freshwater species. Among the more common taxa
are Microcystis aeruginosa, a potential bloom producer, Merismopedia tenuissima, Oscillatoria spp., Dactylococcopsis spp., Chroococcus
spp. and Synechococcus spp. The cyanobacteria are generally considered a nuisance category that do not represent a favorable
food resource, and are commonly associated with increased trophic status. Chlorophytes or green algae, including Ankistrodesmus
falcatus, Chlorella spp., Pediastrum duplex, Scenedesmus acuminatus and Scenedesmus dimorphus are more common from spring to fall
with lowest abundance in winter. Overall, the phytoplankton status in the York has been classified as poor/fair condition. Further
studies are needed regarding interrelationships between the floral and faunal components of the plankton community and link-
ages to water quality and physical environmental factors in the system. In addition, continued observations regarding long-term
trends in phytoplankton abundance and composition need to be followed with emphasis on any increasing presence of potentially
harmful phytoplankton species.
Phytoplankton are the microscopic plant communities
present in water based habitats throughout the world. They
are common components in ponds and lakes of various sizes,
rivers, estuaries and the world oceans. Species within this cat-
egory may vary from less than one micron to several mm in
size, in addition to filamentous forms that are several cm in
length. However, phytoplankton are most common as uni-
cellular taxa, or as colonial species. Their significance is that
they represent a major food source associated with numerous
fauna in these aquatic habitats which they in turn are linked
to other predators, including those leading to the higher tro-
phic levels. Through the process of photosynthesis they are
capable of harvesting solar energy in their transformation of
basic substances in the water to multiply and represent a food
and energy product for various animal species. In addition,
a major bi-product of their photosynthesis is oxygen, which
is released into the water as another essential commodity for
biota in these habitats.
Phytoplankton development will be influenced by the
availability of sunlight and specific nutrients in the water.
However, an excess of these nutrients during favorable condi-
tions for growth may result in a rapid increase in their abun-
dance to produce an algal bloom. This condition is often so
dense that due to the photosynthetic pigments in their cells,
the blooms will be associated with a red or brown coloration in
the water that is often referred to as a “red or mahogany tide.”
The environmental impact of these massive blooms may in-
clude a reduction or depletion of oxygen within these waters.
Although these bloom producing algae normally include au-
totrophic oxygen producing species during daylight hours,
with darkness and the cessation of photosynthesis, their con-
tinual respiratory demands often results in reduced oxygen
levels in late evening hours, and may result in either fish kills,
or general stress conditions among the fauna. The death of
the massive numbers of bloom species and their accumulation
in the sediment will subsequently involve their decomposition
with associated oxygen uptake, also contributing to hypoxic
or anoxic conditions in these waters. Fortunately, the bloom
events are generally short-lived and due to their dissipation
by river flow and tidal action, lower concentrations of these
algae will eventually be re-established.
PHYTOPLANKTON COMPOSITION, ABUNDANCE,
The York River possesses a diverse phytoplankton com-
munity represented by a variety of algal species that includes
both freshwater and estuarine flora. The freshwater species
come from the two major tributaries of the York River (Pa-
munkey River, Mattoponi River) and the streams and marsh-
es bordering the York. A total of 231 taxa was reported for
the Pamunkey River at a tidal freshwater site (Marshall and
Burchardt 2004a), with 254 species recorded within the York
River (Appendix; Marshall, personal records). These spe-
cies are well represented by a diverse assemblage of diatoms,
chlorophytes, cyanobacteria, and cryptomonads, in addition
to dinoflagellates, euglenophytes, and others (Appendix).
Many of the freshwater flora (ca. diatoms, chlorophytes,
cyanobacteria) are abundant in the oligohaline regions,
whereas, the lower reaches of the river remain dominated by
estuarine diatoms and dinoflagellates (Marshall and alden,
1990). This array of species will also change seasonally in the
different regions of the river. There is a natural succession
that begins with a spring flora dominated by several diatom
species, followed by a mixed algal composition in summer and
fall, with a reduced representation and abundance in winter.
The representation of freshwater and estuarine flora in the
York River will be influenced by river flow, tidal movement,
and factors that impact extremes of these events, ca. spring
rains, summer draught, periodic storms, etc. Haas et al. (1981)
also addressed the influence of stratification and mixing to
phytoplankton, with sin et al. (2006) stressing the importance
and control that abiotic conditions (e.g. resource limitation)
have on the phytoplankton presence than biotic factors (pre-
dation). Marshall and Burchardt (2003; 2004a) in a study
of the tidal freshwater Pamunkey stressed the importance of
river flow to phytoplankton composition and productivity.
Since 1985, the composition and abundance of phyto-
plankton in the Pamunkey/York Rivers have been monitored
in the Chesapeake Bay Monitoring Program. Productivity and
autotrophic picoplankton analysis were subsequently added
(e.g. Marshall and alden, 1990; Marshall and affronti,
1992; Marshall and nesius, 1993; Marshall and Burchardt
2003, 2004a, b; 2005; Marshall et al. 2005b). Based on this
data base the mean monthly phytoplankton abundance, total
phytoplankton, biomass, chlorophyll a and productivity over
this entire time period are given for station RET 4.3 in the
York River (Figures 1-4).
The mean monthly phytoplankton concentrations (ex-
cluding the picoplankton) are given in Figure 1. These indi-
cate an extended bi-modal pattern that begins with an early
spring peak (March) that declines into summer. This is a pe-
riod of transition from a major diatom development to a more
diverse representation of taxa in summer that results in a late
summer-early fall development. Lowest concentration will
occur during mid-winter. The mean annual monthly range of
abundance is ca. 5-20 X 106 cells L-1.
Total phytoplankton biomass (which includes autotrophic
picoplankton) is greatest during the spring diatom bloom, de-
creasing into early summer, followed by additional peaks in
summer and autumn (Figure 2). The mean annual monthly
range for algal biomass is ca. 2-10 X 108 pg C L-1. Chlorophyll
Figure 2. Mean monthly total phytoplankton biomass (pg C L-1)
1985-2006, for station RET4.3 in the York River.
Figure 1. Mean monthly phytoplankton abundance (cells/L) 1985-
2006, for station RET4.3 in the York River.
Figure 3. Mean monthly concentrations of Chlorophyll A (µg C L-1)
1985-2006 at station RET4.3 in the York River.
Figure 4. Mean monthly C14 productivity rates (mgC M3 h-1) 1989-
2006, at station RET4.3 in the York River.
a concentrations will also vary over the year (Figure 3). How-
ever, they generally follow the phytoplankton concentrations
with maximum amounts present during early spring and in
summer, with mean monthly values ranging between 7-17 µg
L-1. Phytoplankton productivity is greater between March
and August before decreasing to autumn and winter lows (Fig-
ure 7.4), with mean monthly rates from a January low to a
June high of 13.7 and 79.1 9 mg C M-3 h-1 respectively.
Diatoms are the dominant phytoplankton component
throughout the York River in reference to their diversity,
abundance, and biomass. They are represented by single cell,
or short chain forming series of cells, that represent a major
food source to the various faunal components in these waters.
They are unique in having their cells enclosed within a cell wall
of silica called a frustule, which is composed of two interlock-
ing halves. The dominant freshwater diatoms in these waters
include a variety of pennate (Asterionella formosa) and centric
species (e.g. Aulacoseira granulata, Aulacoseira distans, Cyclotella
meneghiniana (Figure 5), and Skeletonema potamos, among oth-
ers) (Marshall and alden, 1990; Marshall and Burchardt,
trophic and capable of engulfing small prey. There are others
that are mixotrophic. The dinoflagellates are more common
and abundant in the lower segments of the York River where
they have been associated with re-occurring and extensive al-
gal blooms. These include Cochlodinium polykrikoides (Figure
6), Heterocapsa triquetra, Heterocapsa rotundata, Scrippsiella tro-
choidea, and Prorocentrum minimum (Figure 7). Many of these
taxa are associated with “red tide” events in these waters. The
indigenous nature for many of these taxa is enhanced by their
formation of cysts, or “resting” stages, which sink to the sedi-
ment following their motile stage in the water column and
subsequently represent the “seed” population that produce
the motile cells of the next generation of these flora to take
place annually. Many of the dinoflagellates will have maxi-
mum growth periods and corresponding biomass occurring
in early to late spring and again in autumn at concentrations
that are 1-2 X 106 cells L-1. Also there are the sporadic di-
noflagellate blooms common in the lower York. Most con-
spicuous of these is caused by Cochlodinium polykrikoides, which
has produced extensive blooms annually (MackierMan, 1968;
ZuBkoff et al., 1979; Marshall, 1994). In 1992 its abundance
reached 103 cells mL-1 in the York and regions of the lower
Chesapeake Bay, with a massive bloom in the lower York oc-
curring in 2005 that lasted over several days at 103 cells mL-1
(Marshall et al., 2006a).
Figure 5. The diatom Cyclotella meneghiniana.Figure 6. Cochlodinium polykrikoides, a common bloom producing dino-
flagellate in the lower regions of the York River.
Figure 7. The common dinoflagellate Prorocentrum minimum in the
2005). In addition to these common plankton components
in the water column, there are also a variety of taxa associ-
ated with the sediments and are composed of mainly pennate
diatoms, which are also a major food source among the ben-
thos. Many of these benthic species are regularly introduced
into the water column during tidal mixing occasions. Diatoms
will have a bi-modal spring/autumn pattern of development
in the York River with a spring peak occurring in March with
cell abundance ranging 8-18 X 106 cells L-1. The winter low
abundance is ca. 3 X 106 cells L-1). Among the most dominant
species are S. potamos upstream and Skeletonema costatum down-
stream. Diatom biomass values during the year will generally
follow this same pattern as diatom abundance.
These are mainly unicellular species possessing flagella
that allow movement in the water column. Many of these are
autotrophic containing the necessary pigments to allow pho-
tosynthesis to occur, others lacking these pigments are hetero-
Species within this category represent a variety of forms,
and are commonly referred to as blue-green algae. These in-
clude unicellular, colonial, and filamentous taxa that are pre-
dominantly freshwater species. In the York River these taxa
are most common in the upper reaches of river, and in its
two tributaries, with characteristically low abundance in the
higher salinity regions of the river. Among the more com-
mon taxa in the York are Microcystis aeruginosa (Figure 8, a po-
tential bloom producer), Merismopedia tenuissima, plus several
Oscillatoria spp., plus Dactylococcopsis spp., and representative
Chroococcus spp. and Synechococcus spp. The cyanobacteria are
generally considered a nuisance category that do not repre-
sent a favorable food resource, and is commonly associated
with increased trophic status. Their major development in the
York occurs during summer and early autumn at ca. 3-8 X 106
cells L-1 before decreasing into winter months, with their total
cell biomass representation following a similar pattern.
with their maximum development during the summer-early
fall months with concentrations of ca. 2-4.5 X 108 cells L-1.
Their concentrations decline into autumn, with lowest levels
during winter and spring. Their development during summer
is a major contributor to the overall algal productivity, oxygen
production, and food source for a variety of microorganisms.
OTHER CATEGORIES OF PHYTOPLANKTON
In addition to the more dominant flora mentioned above
there are also a variety of background species that season-
ally appear in lesser abundance and biomass, yet contribute
to the overall photosynthetic activity and represent an addi-
tional food and oxygen source. The most common of these
would be the cryptophytes, composed of a variety of motile
single cell taxa present the entire year with mean monthly
concentrations of ca. 1-3 X 106 cells L-1, with peak concentra-
tions during summer and autumn. These taxa include Cryp-
tomonas erosa and Rhodomonas minuta. This group is a suitable
food source for many of the heterotrophic dinoflagellates and
zooplankton. Other algal categories are more frequently as-
sociated with the period following the spring diatom pulse
and occur in summer and early autumn. For instance, the
euglenophytes represent a category often showing pulses of
significant size (3-4 X 104 cells L-1), but are generally in low
abundance. Upstream they include several Euglena spp., with
Eutreptia lanowii more common downstream. Trachelomonas,
and Phacus species are rare within the York. The same can
be said of other eukaryotes that generally play a minor role in
the phytoplankton dynamics in the river.
Among the different phytoplankton categories are also
species that are considered harmful to other biota, or even
be associated with human illness. Several are linked to tox-
in production, et al. related to anoxic or hypoxic conditions
associated with bloom production (Marshall et al., 2005).
Examples of these potentially harmful species include the
dinoflagellates Akashiwo sanguinea, Cochlodinium polykrikoi-
des, Dinophysis acuminata, Karlodinium micrum, Prorocentrum
minimum, Pfiesteria piscicida, Pfiesteria shumwayae; the diatom
Pseudo-nitzschia seriata; the cyanobacteria Microcystis aeruginosa,
among others (See Marshall et al., 2005a for list of 34 taxa).
Within the York River attention has recently been focused on
increasing concentrations and any associated environmental
impact related to blooms of the dinoflagellates Cochlodinium
polykrikoides, Karlodinium micrum, and Prorocentrum minimum.
STATUS AND TRENDS
Using a 16-year database for stations in the Pamunkey/York
River several significant long term phytoplankton trends have
been identified in addition to several water quality variables
(Marshall and Burchardt, 2004b). Increasing trends in total
phytoplankton abundance and biomass were indicated along
with similar increasing biomass trends for the diatoms, cyano-
bacteria, chlorophytes, and cryptomonads. There was a nega-
tive trend associated with the autotrophic picoplankton, with
none indicated for the dinoflagellates. Of note, other trends
included increasing TP concentrations, and decreasing TN:TP
ratios (ca. 11.0). In this analysis there were also decreasing
trends in Secchi readings matched with increasing levels of TSS.
A further appraisal of the York River phytoplankton habi-
tats was included in the paper by Lacouture et al. (2006). They
Figure 8. Microcystis aeruginosa, a colonial forming species of the cya-
These are common freshwater species, commonly known
as green algae. Their high concentrations in the York River
are more limited to the low salinity areas below the confluence
of the Pamunkey and Mattoponi Rivers, but would increase in
abundance downstream during high river flow. Their pres-
ence normally diminishes downstream. Common representa-
tion in the water column would be by Ankistrodesmus falcatus,
Chlorella spp., Pediastrum duplex, Scenedesmus acuminatus and
Scenedesmus dimorphus. Chlorophytes are more common from
spring to fall with lowest abundance in winter. Their concen-
tration levels are generally between 0.3-0.8 X 106 cells L-1 and
usually these represent a small fraction of the algal biomass
that would peak in summer.
This is a special phytoplankton category composed of cells
less than 2 microns in size. The populations are composed of
mainly single cell or colonial cyanobacteria, and to a much
lesser representation by chlorophytes and other eukaryotes.
Autotrophic picoplankton are ubiquitous throughout the year
developed a phytoplankton index of biotic integrity based
on a community structure protocol described by Buchanan
et al. (2005), and using an 18-year data set coming from the
Chesapeake Bay Phytoplankton Monitoring Program. This
approach utilized a combination of nutrients (DIN, PO4) and
Secchi depth values to characterize the phytoplankton habi-
tat conditions at sites in the Chesapeake Bay and several of
its major tributaries within a variety of salinity ranges during
spring and summer. A variety of phytoplankton metrics were
chosen to provide a ranking for these locations (e.g. Poor, Fair,
Good). In the characterization for the upper-river and lower
river mouth sites in the York River, both received a spring
status ranking of poor/fair, and in summer poor and poor/fair
respectively. However, it should be noted that many of the
sites in the Chesapeake Bay Monitoring Program included
rankings of Poor and Poor/fair, with a Good ranking rare. A
Poor (impaired) status was interpreted as having an excess of
DIN or PO4 levels and reduced water clarity that would be as-
sociated with the degree and composition of phytoplankton
development at these locations. A Fair classification would
represent an improved condition in one of these variables.
Considering this classification, an increase in nutrient levels
within the York would not be considered desirable for the en-
vironmental status in the York. Thus, although many of the
phytoplankton trends are presently favorable, a continued in-
crease in nutrient levels may easily end this pattern and pro-
duce a variety of less favorable species for food and oxygen
production (including others that are potentially harmful)
within the York River.
FUTURE RESEARCH NEEDS
Further studies are needed regarding interrelationships
between the floral and faunal components of the plankton
community and linkages to water quality and physical envi-
ronmental factors within the various salinity regions and tro-
phic levels in the system. In addition, continued observations
regarding long-term trends in phytoplankton abundance and
composition need to be followed with emphasis on any in-
creasing presence of potentially harmful phytoplankton spe-
cies. Each of these areas are linked to various important fin
fish and shellfish resources utilized in the river and would be
associated with their harvest and related socio-economic con-
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York River Phytoplankton Species List
Aulacoseira granulata var. angustissima
Biddulphia rhombus f. trigona
Chaetoceros didymus var. protuberans
Chaetoceros socialis lauder
Coscinodiscus oculus iridis
Gyrosigma balticum silimis
Navicula cuspidata var. ambigua
Proboscia alata gracillima
Gymnodinium sp. <20 microns
Gymnodinium sp. >20 microns
Ankistrodesmus falcatus var. mirabilis